1. Field of the Invention
The present invention relates to printed circuit manufacturing techniques. More particularly, it relates to additive plating on polymer or ceramic thick film circuitry patterns.
2. Description of the Prior Art
Conventional circuit manufacturing processes for printed circuit boards involve the use of subtractive processes in which several steps take place, including activation, sensitization, electroless and electrolytic plating, photo imaging, and etching. As a result of carrying out these steps, significant amounts of waste materials are generated and the handling thereof directly increases overall cost of manufacturing.
Recently industry has considered additive circuitizing techniques in order to minimize chemical waste handling, reduce overall costs, and improve product yield. Using this additive approach, a conductive polymer paste is used to print circuit patterns on the substrate. Thereafter, generally electroless plating is done. The need for electroless plating proceeds from the presence of a thin oxide layer on the metallic fillers and a possible skin effect by the polymer binder.
This situation again results in excessive chemical waste and potential reliability problems, such as adhesion failure due to the high operating temperatures associated with most electroless plating baths and processes.
Additional problems may arise in the nature of film degradation due to the possibility of chemical reactions between the thick film polymer binder and conventionally high alkaline electroless plating bath solutions.
These problems are primary reasons for the limited application and utilization of additive processes in electrical circuitry manufacture. At present, such conductive polymer additive circuitizing is used in low power applications such as consumer products.
U.S. Pat. No. 4,270,985 to Lipson et al relates to screen printing photopolymerizable inks. There is a teaching of electroplating screen printed substrate in a bath containing copper sulfate, sulfuric acid and a brightener, for thirty minutes at 25 degrees centigrade, with an applied current of 30 amperes per square foot. Lipson et al discloses several electroplating baths, one of which is an acid copper bath containing 215 grams per liter of sulfuric acid, which is a very high concentration, so high as to adversely affect a polymer thick film. A current density of 30 amps per square foot is also extremely high due to the inherent electrical resistance of polymer thick films, for use therewith. Further, the disclosed surface conditioning, plating and etching steps preclude the use of certain circuit substrates such as anodic alumina and similar materials sensitive to high alkaline solutions.
U.S. Pat. No. 3,769,179 to Durose et al relates to a copper plating process for perforated printed circuit boards and discloses a high acid, low copper bath, preferably including one or more grain refining agents. A high acid content would degrade the polymer binder if the bath were used to plate on polymer thick films. However, U.S. Pat. No. 4,242,181 to Malak also relates to copper plating printed circuit boards and discloses improvements to the Durose et al high acid, low copper, electroplating bath wherein coffee is added to the bath as a grain refining agent.
U.S. Pat. No. 3,923,613 to Immel relates to an acidic, galvanic, high luster copper bath, including organic sulfonic acids or the salts thereof and/or non-ionogenic wetting agents, the improvement provided by adding urea thereto.
It is an object of the present invention to provide a plating bath suitable for plating polymer thick film materials. Such a bath requires a special chemistry which does not degrade the polymer binder and yet is aggressive enough to remove any oxide layered on the metallic filler.